In general, a refrigerator is composed of at least a compressor, a condenser, an expansion mechanism (such as an expansion valve), and an evaporator, or furthermore by a dryer, and has a structure that a mixed liquid of a refrigerant and a lubricant oil (i.e., a refrigerator oil) is circulated in the closed system. As the refrigerant for a refrigerator, a chlorine-containing compound, such as a chlorofluorocarbon, has been widely used, but in consideration of the environment protection, it is being replaced by a compound containing no chlorine, such as a hydrofluorocarbon (HFC), and a natural refrigerant, such as carbon dioxide. Examples of the hydrofluorocarbon in practical use include a saturated hydrofluorocarbon (which may be hereinafter referred to as a saturated HFC), which is represented by 1,1,1,2-tetrafluoroethane, difluoromethane, pentafluoroethane, and 1,1,1-trifluoroethane (which are referred to as R134a, R32, R125, and R143a, respectively).
Various investigations have been made on a refrigerator oil composition that is suitable for a saturated HFC and a carbon dioxide refrigerant. For example, PTL 1 discloses that a refrigerator oil composition in which a polyhydric alcohol partial fatty acid ester, and a phosphate compound and so on are blended to at least one selected from a polyoxyalkylene glycol compound and an ester compound. PTL 1 shows that the use of the refrigerator oil composition enhances the wear-resistant properties and the lubricating performance while increasing the compatibility of the refrigerator oil with a saturated HFC.
Furthermore, for example, PTL 2 discloses that a refrigerator oil composition for a carbon dioxide refrigerant, in which a carbonyl carbonate derivative and/or a polyol ester is blended to a polyoxyalkylene glycol, thereby improving the compatibility with a carbon dioxide refrigerant and also enhancing the wear-resistant properties and the lubricating performance.
However, under the current situation, carbon dioxide, which requires high pressure, cannot be used as a major component of a refrigerant for a car air-conditioner, and a saturated HFC is being avoided due to the high global warming potential thereof. In recent years, the use of an unsaturated fluorinated hydrocarbon compound, such as HFO1234ze and HFO1234yf, is being considered as a refrigerant that is capable of being used for the current car air-conditioner system, irrespective of the low global warming potential thereof.
Examples of the refrigerator oil that is used for a refrigerant composed of an unsaturated fluorinated hydrocarbon compound include oxygen-containing compounds, such as a polyoxyalkylene glycol compound, a polyol ester compound and a polycarbonate compound (see, for example, PTL 3). Among these base oils, the polyoxyalkylene glycol compound is excellent in thermal stability and is being studied for the purpose in an open type car air-conditioner. The polyol ester compound is being mainly studied for the purpose in an electric car air-conditioner equipped with a motor therein due to the high volume resistivity thereof.